I am asked to design a replicated OpenLDAP implementation for use on
1500 of our customers servers who are now use a non-replicated
configuration using the standard passwd/shadow backend combined with a
PostgreSQL database. Our customers consist of primary schools and will
use the database for authentication through Samba. The reason we want to
replicate the data is so that we can offer email and other services from
a central datacentre.

If every school has it own tree in the DIT, like
ou=school-1,o=your organization,c=nl

and you have no need to write directly to the directory except from the
school itself (write in central could use referrals to school), you
could set up the school ldap as syncprovider and replicate to a glued
database of 1500 school-databases (consumer databases) in the datacenter
where you have the whole DIT altogether